Nuclear heating is in particular induced by the radiation of photons and neutrons that exist within a nuclear reactor. The measurement of nuclear heating allows the field of radiation in a nuclear reactor (core and reflector/periphery) to be indirectly accessed.
The deposition of energy per unit weight and time (W/g) by radiative interactions (neutrons and photons)/matter is called nuclear heating.
Neutron radiation and more precisely the number of neutron particles, also called neutron fluence or flux, may be quantified using specific systems such as fission chambers, self-powered detectors or even activation detectors. Photon radiation may for its part be quantified using specific systems such as ionization chambers or self-powered detectors. Neutron and photon radiation has the property of depositing its energy into matter, and therefore of heating it. By matter, what is meant is matter contained in nuclear reactors, for example the structures of the reactor, experimental devices, and any materials present in the core of the reactor (including the nuclear fuel).
Mention is made of quantifying the amount of overall nuclear heating produced by the radiation, rather than the flux of particles, in order to determine the effect of this radiation.
This is particularly appropriate in experimental nuclear reactors in which the internal structures and systems (experimental devices for example) differ depending on the experimental channel and depending on the experimental program.
It is all the more necessary to measure nuclear heating in experimental nuclear reactors because it is a key quantity that must be taken into account when dimensioning experimental devices, in particular from the point of view of their mechanical strength, and their thermal withstand.
Nuclear heating is conventionally measured by a calorimetric method. Calorimetric methods essentially consist in determining the nuclear heating of a small piece of material, or sample, the mass of which is known by measurement of one or more temperature increases or one or more temperature differences.
In the rest of the description, this small piece of material will be referred to as the sample.
The sample is conventionally made of graphite.
The temperature increase or temperature difference may be due to photon and neutron radiation. It may also be due, possibly in combination with radiation, to a heating system integrated into the calorimeter, for example for the purpose of calibrating the calorimeter outside of the reactor or of implementing, in a reactor, the measurement protocols said to be “de zéro” and “d'addition de courant”. Such measurement protocols are described in the patent FR 2 968 448.
A differential calorimeter is commonly used. In this case, the calorimeter comprises two sample holders. The measurement of nuclear heating with a differential calorimeter is based on a double temperature difference between two essentially identical sample holders, a first sample holder being full, i.e. containing a sample of material for which the deposited energy must be measured, and a second empty sample holder serving as reference. The deposited energy is deduced from this double temperature difference between the two sample holders, and conventionally expressed in W/g. The temperatures may be measured by thermocouples.
One type of sample holder and one type of differential calorimeter are described in the publication “Nuclear Heating Measurements in Material Testing Reactor: a Comparison Between a Differential calorimeter And a Gamma Thermometer, D. Fourmentel, C. Reynard-Carette, A. Lyoussi, J. F. Villard, J. Y. Malo, M. Carette, J. Brun, P. Guimbal, Y. Zerega, IEEE Transactions on Nuclear Science, Volume 60, Issue:1, Part:2, Publication Year: 2013, Page(s): 328-335”.
The differential calorimeter is a non-adiabatic calorimeter insofar as heat is exchanged between the calorimeter and a heat-transfer fluid exterior to the calorimeter. It comprises two sample holders.
Each sample holder comprises 3 portions: a head, a base and a rod axially connecting the head and the base. The 3 portions lying longitudinally on the same axis. A first thermocouple is located at the base of the head, level with its link with the rod. A second thermocouple is located in the middle of the base.
Another type of sample holder is described in the publication “Principle of calibration of the simple calorimeter for nuclear heating measurements in MARIA reactor and transposition to the case of JHR reactor., M. Tarchalski, K. Pytel, P. Siréta, A. Lyoussi, J. Jagielski, C. Reynard-Carette, C. Gonnier, G. Bignan, ANIMMA 2013, 23-27 June, Marseille, France, ISBN: 978-1-4799-1046-5”.
The sample holder contains a cylindrical central sample mounted in a stainless steel casing. A gas-filled space is provided between the cylindrical sample and the casing. A thermocouple is inserted into the center of the central sample. Another thermocouple is fastened to the exterior of the casing. The temperature difference is measured between the two thermocouples.
Beyond a certain level of deposited energy, the sample holder according to this publication does not allow heat to be removed from the sample because a layer of insulating gas surrounds the sample (high induced temperatures). Specifically, FIG. 2 of this publication shows that temperature decreases radially essentially to the level of the layer of gas. To remove high levels of deposited energy, it is necessary to decrease the thickness of the layer and/or modify the nature of the gas.
In both the aforementioned publications, the sample holders and calorimeters do not promote radial heat exchange.
One objective of the invention is to provide, in response to this problem, a sample holder and a calorimeter cell comprising at least one such sample holder.